Optical multi/demultiplexer, optical multi/demultiplexing method, and optical filter

ABSTRACT

Disclosed is an optical multi/demultiplexer, which comprises a graded index type light transmission section whose refractive index is continuously reduced in the direction from the center to the periphery thereof, a wavelength-multiplexed-light connecting section disposed at one of the axial ends of the light transmission section, and at least one monochromatic light connecting section disposed at the other axial end of the light transmission section. The wavelength-multiplexed-light connecting section and the monochromatic light connecting section are disposed relative to the light transmission section in such a positional relationship that a monochromatic light included in a wavelength multiplexed light entered from the wavelength-multiplexed-light connecting section into the one end of the light transmission section is emitted from the other end of the light transmission section at a position where the monochromatic light connecting section is disposed. The present invention can provide an optical multi/demultiplexer having a reduced number of components and allowing the number of assembling steps to be reduced while eliminating the need for complicated adjustments.

TECHNICAL FIELD

The present invention relates to an optical multi/demultiplexer, anoptical multi/demultiplexing method, and an optical filter (opticalswitch) capable of picking up a light having a specific wavelength.

BACKGROUND ART

Late years, in connection with the advent of the multimedia informationsociety with the popularization of portable telephones, motion picturecommunications, electronic commerce and others, the transmission volumeof communication networks is being continuously increased. As atechnique capable of sending and receiving a large amount of informationcommunication at a lower cost, there has been employed a transmissionsystem based on a wavelength division multiplexing transmissiontechnology comprising giving different informations respectively, tolights different in wavelength, multiplexing the lights with thedifferent wavelengths, and transmitting the wavelength multiplexed lightthrough a single optical fiber. This transmission system uses a filterelement for multiplexing/demultiplexing light, an optical filter orswitch for selectively picking up a light having a specific wavelengthfrom a wavelength multiplexed light including a plurality ofwavelengths, and other components. As one of the filter elements for usein the above system, there has been known an optical demultiplexer asshown in FIG. 1. In this optical demultiplexer, a plurality ofinterference film filters 101, 102, 104 adapted, respectively, toreflect lights of different wavelengths are disposed on the optical pathof a wavelength multiplexed light 106 transmitted through an opticalfiber, so as to reflect lights of specific wavelengths λ1, λ2, λ3included in the wavelength multiplexed light 106, respectively, toobtain monochromatic lights 108, 110, 112 of specific wavelengths λ1,λ2, λ3.

There has also been known an optical demultiplexer as shown in FIG. 2,which comprises in combination a star coupler 201 and a plurality ofinterference film filters 202, 204, 206, 208. In this opticaldemultiplexer, the star coupler 201 for branching light is disposed onthe optical path of a wavelength multiplexed light 210 transmittedthrough an optical fiber, and the interference film filters 202, 204,206, 208 adapted, respectively, to allow only lights of wavelengths λ1,λ2, λ3, λ4 to be transmitted therethrough are disposed on correspondingbranched optical paths.

When the above optical demultiplexers are used in such a manner that thedirection of the optical path thereof is reversed, they can act as anoptical multiplexer for multiplexing monochromatic light flux.

The above optical demultiplexer, optical multiplexer, ormulti/demultiplexer essentially includes filters of the same number asthat of multiplexed wavelengths. Thus, they involve a high-cost problemdue to increase in the number of components and the number of assemblingsteps, and the need for complicated adjustments. Further, the opticaldemultiplexer using a star coupler involves a problem of degradation inoutput light intensity due to branching of a multiplexed light throughthe star coupler.

As optical filters or optical switches, there have been developed anoptical filter having a grating disposed in a Mach-Zehnderinterferometer type optical waveguide, and a thermooptic optical switchprovided with a thin-film heater which is composed of a thin layer ofmetal, such as a chromium or copper, vapor-deposited on the aboveoptical filter, and designed to generate heat in response to currentsupplied thereto to heat the optical waveguide.

However, the optical filter and the optical switch having theabove-described optical waveguide is not suited for mass production dueto its complicated production process. Further, since it is required inthe process of forming the grating to periodically form notches in acore or clad with a high precision in the order of several microns ormore, the cost is increased.

In view of the above circumstances, it is therefore an object of thepresent invention to provide an optical multi/demultiplexer having areduced number of components and allowing the number of assembling stepsto be reduced while eliminating the need for complicated adjustments.

It is another object of the present invention to provide an opticalfilter capable of being readily produced and varying the wavelength of amonochromatic light to be picked up.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedan optical multiplexer comprising a graded index type light transmissionsection whose refractive index is continuously reduced in the directionfrom the center to the periphery thereof, a wavelength-multiplexed-lightconnecting section disposed at one of the axial ends of the lighttransmission section, and at least one monochromatic light connectingsection disposed at the other axial end of the light transmissionsection. The wavelength-multiplexed-light connecting section and themonochromatic light connecting section are disposed relative to thelight transmission section in such a positional relationship that amonochromatic light included in a wavelength multiplexed light enteredfrom the wavelength-multiplexed-light connecting section into the oneend of the light transmission section is emitted from the other end ofthe light transmission section at a position where the monochromaticlight connecting section is disposed.

In a preferred embodiment of the present invention, the lighttransmission section is made of plastic. In another preferred embodimentof the present invention, the materials of the light transmissionsection are different in Abbe number at 20 or more.

In still another preferred embodiment of the present invention, thelight transmission section has an substantially cylindrical body, andthe wavelength-multiplexed-light connecting section is disposed to theradially peripheral region of the light transmission section and inparallel with the axis of the light transmission section.

In yet another preferred embodiment of the present invention, the lighttransmission section has an substantially cylindrical body, and thewavelength-multiplexed-light connecting section is disposed to theradially central region of the light transmission section while beinginclined at an angle close to the aperture angle of the lighttransmission section.

In yet still another preferred embodiment of the present invention, themonochromatic light connecting section is provided in a plural number.

According to a second aspect of the present invention, there is providedan optical demultiplexing method comprising; entering a wavelengthmultiplexed light formed by multiplexing a plurality of monochromaticlights, from one of the axial ends of a graded index type lighttransmission section whose refractive index is continuously reduced inthe direction from the center to the periphery thereof; demultiplexingthe wavelength multiplexed light into the monochromatic lights in thelight transmission section; and emitting the monochromatic lights,respectively, from different positions of the other axial end of thelight transmission section.

According to a third aspect of the present invention, there is providedan optical multiplexing method comprising: entering a plurality ofmonochromatic lights, respectively, from different positions of one ofthe axial ends of a graded index type light transmission section whoserefractive index is continuously reduced in the direction from thecenter to the periphery thereof; multiplexing the monochromatic lightsinto a wavelength multiplexed light in the light transmission section;emitting the wavelength multiplexed light from the other axial end ofthe light transmission section.

According to a fourth aspect of the present invention, there is providedan optical filter comprising a graded index type light transmissionsection whose refractive index is continuously reduced in the directionfrom the center to the periphery thereof, a wavelength-multiplexed-lightconnecting section disposed at one of the axial ends of the lighttransmission section, and a monochromatic light connecting sectiondisposed at the other axial end of the light transmission section.

In a preferred embodiment of the present invention, the lighttransmission section has a refractive index profile constant to bevaried according to temperature, and the optical filter further includestemperature varying means for varying the temperature of the lighttransmission section to vary the positions in the other axial end of thelight transmission section where a plurality of monochromatic lightsincluded in a wavelength multiplexed light entered from thewavelength-multiplexed-light connecting section-into the lighttransmission section are to be emitted, respectively, therefrom, so asto allow either one of the monochromatic lights to be selectivelyentered into the monochromatic light connecting section.

In the optical filter having the above structure, the wavelength of themonochromatic light to be introduced into the monochromatic connectingsection can be varied by varying-the temperature of the lighttransmission section. Thus, a desired wavelength of light can beselectively picked up with a simple structure.

In another preferred embodiment of the present invention, the lighttransmission section is made of plastic. In still another preferredembodiment of the present invention, the refractive index profile of thelight transmission section has a temperature dependent constant of5×10⁻⁵ or more. In yet another preferred embodiment of the presentinvention, the light transmission section has an substantiallycylindrical body, and the wavelength-multiplexed-light connectingsection is connected to the light transmission section in parallel withthe axis of the light transmission section. In yet still anotherpreferred embodiment of the present invention, the temperature varyingmeans includes a Peltier element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a conventional opticaldemultiplexer.

FIG. 2 is a schematic diagram showing another conventional opticaldemultiplexer.

FIG. 3 is an explanatory schematic side view of the structure andfunction of an optical demultiplexer according to a first embodiment ofthe present invention.

FIG. 4 is a schematic side view showing one modification of the opticaldemultiplexer according to the first embodiment of the presentinvention.

FIG. 5 is a schematic perspective view showing a photodetector array foruse in the optical demultiplexer in FIG. 4.

FIG. 6 is an explanatory diagram of the principle of the opticaldemultiplexer according to the first embodiment of the presentinvention.

FIG. 7 is an explanatory diagram of the effect of the opticaldemultiplexer according to the first embodiment of the presentinvention.

FIG. 8 is a schematic perspective view showing an optical filteraccording to a second embodiment of the present invention.

FIG. 9 is an explanatory schematic side view of the structure andfunction of an optical filter according to a third embodiment of thepresent invention.

FIG. 10 is an explanatory diagram of the principle of the optical filteraccording to the third embodiment of the present invention.

FIG. 11 is a graph showing the temperature dependence of the refractiveindex profile of a transmission section in the optical filter accordingto the third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the drawings, a preferred embodiment of the presentinvention will now be described in detail. FIG. 3 is a schematic diagramshowing an optical demultiplexer 1 according to a first embodiment ofthe present invention. As shown in FIG. 3, the optical demultiplexer 1includes a light demultiplexing section 1. This light demultiplexingsection 2 has an substantially cylindrical shape, and serves as a gradedindex type light transmission section whose refractive index iscontinuously reduced in the direction from the axis c to the peripherythereof In the first embodiment, the refractive index n of the lightdemultiplexing section 2 has a distribution approximated by thefollowing formula (1):n(r)=n ₀×(1−g ² ×r ²/2)   (1),wherein n₀: refractive index on the axis c,

-   -   g: refractive index profile constant, and    -   r: radial distance from the axis c.

For example, the light demultiplexing section 2 is a light transmissionsection made of inorganic glass through an ion exchange process, or alight transmission section made of plastic. The light demultiplexingsection 2 has a periphery covered with a cladding layer (not shown) toallow a light entered from one (left side in FIG. 3, hereinafterreferred to as “first axial end”) of the axial ends thereof to bepropagated through the inside thereof and emitted from the other axialend (right side in FIG. 3, hereinafter referred to as “second axialend”).

As described later, the light demultiplexing section 2 in the firstembodiment is used to multiplex and demultiplex light by utilizing thewavelength dispersion characteristic of the light demultiplexing section2 such that it has different refractive index profiles with respect toeach of different wavelengths of light. For this purpose, the lightdemultiplexing section 2 is preferably made of materials largelydifferent in Abbe number to increase the difference in refractive indexprofile with respect to each of different wavelengths of light. Thedifference in Abbe number is preferably 20 or more, more preferably 50or more. While it is preferable to set the difference in Abbe number aslarge as possible, it is typically set at 80 or less.

The optical demultiplexer 1 includes an incidence-side connectingsection 8 serving as a wavelength-multiplexed-light connecting sectionfor allowing an optical fiber 6 transmitting a wavelength multiplexedlight 4 to be connected thereto, at the first axial end of the lightdemultiplexing section 2. Preferably, the incidence-side connectingsection 8 is disposed in the peripheral region of the tightdemultiplexing section in substantially parallel with the axis c of thelight demultiplexing section 2 to allow a light to be entered into theperipheral region (radially peripheral region) in substantially parallelwith the axis of the light demultiplexing section 2.

A glass optical fiber made of silica glass, and a plastic optical fibermade of material such as polymethylmethacrylate, polystyrene orpolycarbonate can be used as the optical fiber 6 to be connected to theoptical demultiplexer 1.

The optical fiber 6 is not limited to one having a specific refractiveindex profile, but may be a conventional optical fiber, such asstep-index (SI) type of optical fibers or graded-index type (GI) opticalfibers.

The optical demultiplexer 1 also includes an outgoing-side connectingsection serving as a monochromatic light connecting section for allowinga monochromatic light to be emitted therefrom, at the second axial endof the light demultiplexing section 2. In the first embodiment, theoutgoing-side connecting section is formed in a prism 10 made oftransparent resin. The prism 10 is connected to the second axial end ofthe light demultiplexing section 2 in such a manner that three types ofmonochromatic lights different in wavelength (λ1, λ2, λ3) are entered tothe prism 10 from the different positions (r21, r22, r23) in the secondaxial end of the light demultiplexing section 2, respectively. Thesurface of the prism 10 for emitting the monochromatic lights enteredfrom the light demultiplexing section 2 has three of the outgoing-sideconnecting sections 18, 20, 22 for allowing three optical fibers 12, 14,16 to be connected thereto in such a manner that the monochromaticlights emitted from the prism 10 are entered into the optical fibers 12,14, 16, respectively. Therefore, each of these optical fibers may havethe outgoing-side connecting section.

The incidence-side connecting section 8 and the outgoing-side connectingsections 18, 20, 22 are not limited to a specific structure, but anysuitable conventional structure may be applied thereto. For example,these connecting sections may be composed of a transparent resin filledbetween the light demultiplexing section 2 and either one of a lightsource for emitting a wavelength multiplexed light, a light-receivingelement for receiving a monochromatic light and an optical fiber.

Instead of the prism 10, a resin material itself or a resin product forforming the outgoing-side connecting section may be disposed at thesecond axial end of the light demultiplexing section 2. Further, asshown in FIG. 4, instead of the optical fibers 12, 14, 16, aphotodetector array 26 (see FIG. 5) having a plurality of arrayedlight-receiving elements (photodetectors) may be used to receive theemitted monochromatic lights. In this case, the light-receiving elements24 are disposed, respectively, at positions corresponding to thepositions where the monochromatic lights are emitted therefrom. That is,the light-receiving elements 24 serve as the outgoing-side connectingsections.

The optical demultiplexer 1 according to the first embodiment isoperable to demultiplex the wavelength multiplexed light 4 formed bymultiplexing the three types of monochromatic lights different inwavelength (λ1, λ2, λ3) into the monochromatic lights having thewavelengths λ1, λ2, λ3, respectively. In the optical demultiplexer 1, alight entered from the incidence-side connecting section 8 into thelight demultiplexing section 2 is propagated through the lightdemultiplexing section 2 in a meandering manner in a given cycle. Thelight demultiplexing section 2 has the refractive index n continuouslyreduced in the direction from the axis c to the periphery thereof. Thus,if the entered light is a wavelength multiplexed light, each lightsdifferent in wavelength, which are included in the wavelengthmultiplexed light, will be separately propagated through different pathsbecause the refractive index profile constant is varied according to thedifference in wavelength. Thus, as shown in FIGS. 3 and 4, if theentered light is a wavelength multiplexed light, the monochromaticlights will be propagated, respectively, in different meandering cyclesdepending on the wavelengths (λ1, λ2, λ3) thereof, and emitted,respectively, from the different positions (r21, r22, r23) of the lightdemultiplexing section 2.

A light matrix in the light demultiplexing section 2 having therefractive index profile approximated by the formula (1) is expressed bythe following formula (2): $\begin{matrix}{{\begin{pmatrix}{r2} \\{\theta 2}\end{pmatrix} = {\begin{pmatrix}{\cos({gZ})} & {{\sin({gZ})}/{ng}} \\{{- {ng}} \cdot {\sin({gZ})}} & {\cos({gZ})}\end{pmatrix}\begin{pmatrix}{r1} \\{\theta 1}\end{pmatrix}}},} & (2)\end{matrix}$,wherein n1: refractive index on the axis c,

-   -   g1: refractive index profile constant,    -   Z: length of the light demultiplexing section 2,    -   r1: incidence position where a light 28 is entered into the        first axial end of the light demultiplexing section 2,    -   θ1: incidence angle (rad) at which the light 28 is entered into        the first axial end of the light demultiplexing section 2,    -   r2: outgoing position where the light 28 is emitted from the        second axial end of the light demultiplexing section 2,    -   θ2: outgoing angle (rad) at which the light 28 is emitted from        the second axial end of the light demultiplexing section 2 (see        FIG. 6).

As seen in the formula (2), the outgoing position of the light in theoutgoing end (second axial end) is changed depending on the refractiveindex profile constant g and the refractive index n. Thus, as shown inFIG. 3, when a plurality of lights different in wavelength (λ1, λ2, λ3)are entered from the same incidence position (same radial position) inthe first axial end of the light demultiplexing section 2, the lightsare emitted, respectively, from the different outgoing positions (r21,r22, r23) in the second axial end of the light demultiplexing section 2because the refractive index profile constant g and the refractive indexn are different with respect to each of the lights.

The optical demultiplexer 1 de multiplexes the wavelength multiplexedlight 4 by utilizing this phenomenon. Thus, in the optical demultiplexeraccording to the first embodiment, the position r (distance from theaxis c) of the incidence-side connecting section 8 and the positions ofthe outgoing-side connecting sections 18, 20, 22 are arranged in such apositional relationship that the wavelength multiplexed light 4 formedby multiplexing the three monochromatic lights different in wavelength(λ1, λ2, λ3) and entered into the light demultiplexing section 2 throughthe incidence-side connecting section 8 is demultiplexed in the lightdemultiplexing section 2 based on the difference in refractive indexwith respect to each of the wavelengths, and the demultiplexedmonochromatic lights of the wavelengths (λ1, λ2, λ3) are entered,respectively, into the three optical fibers 12, 14, 16 through theoutgoing-side connecting sections 18, 20, 22.

The incidence-side connecting section 8 is disposed in the peripheralregion of the light demultiplexing section 3 to allow the light to beentered into the light demultiplexing section 2 from the peripheralregion, so that the respective optical paths for the wavelengths can belargely separated from each other. Thus, the outgoing positions r21,r22, r23 of the monochromatic lights and the positions of theoutgoing-side connecting sections 18, 20, 22 can be spaced apart fromeach other to facilitate the detection of each of the monochromaticlights emitted from the light demultiplexing section 2. From this pointof view, when a wavelength multiplexed light is entered into the lightdemultiplexing section 2 in parallel with the axis thereof, the positionof the incidence-side connecting section 8 relative to the lightdemultiplexing section 2 is preferably arranged on the side of theperiphery as close as possible. Otherwise, when the incidence-sideconnecting section 8 is located at or adjacent to the axis (radiallycentral region) of the light demultiplexing section 2, theincidence-side connecting section 8 is preferably inclined relative tothe axis of the light demultiplexing section 2 at an angle close to theaperture angle of the light demultiplexing section 2 to enter a lightinto the light demultiplexing section 2 at the aperture angle of thelight demultiplexing section 2.

For example, in the optical multiplexer 1 arranged as above, thediameter and the length (Z) of the light demultiplexing section 2 areset at 1 mm and 5.37 mm, respectively. Then, a wavelength multiplexedlight formed by multiplexing three monochromatic lights different inwavelength (λ1=470 nm, λ2=525 nm, λ3=630 nm) are entered from a positionaway from the center c of the first end of the light demultiplexingsection 2 at a distance of 0.45 mm (or r=0.45 mm). Given that therefractive index profile constants g₁, g₂ and g₃ of the lightdemultiplexing section 2 with respect to the wavelengths λ1, λ2 and λ3are 0.902, 0.891 and 0.878, respectively, and the refractive indices n₁,n₂ and n₃ on the axis c with respect to the wavelengths λ1, λ2 and λ3are 1.518, 1.513 and 1.510, respectively. The monochromatic lightshaving the wavelengths (λ1=470 nm, λ2=525 nm, λ3=630 nm) are emitted,respectively, from the different positions in the second axial end ofthe light demultiplexing section 2 (see FIG. 3). Specifically, themonochromatic light of the wavelength λ1, the monochromatic light of thewavelength λ2, and the monochromatic light of the wavelength 73 areemitted from the position r21 away from the center c at a distance of0.06 mm, the position r22 away from the center c at a distance of 0.03mm and the position r23 on the center c in the second axial end of thelight demultiplexing section 2, respectively, to exhibit a spectradistribution having the intensity 1 as shown in FIG. 7. Thus, thedemultiplexed monochromatic lights can be obtained at the outgoing-sideconnecting sections 18, 20, 22.

In the optical demultiplexer 1 according to the first embodiment,monochromatic light sources of wavelengths λ1, λ2, λ3 are disposed,respectively, at the monochromatic light (outgoing-side) connectingsections 18, 20, 22 to use these outgoing-side connecting sections asincidence-side connecting sections, and a photodetector is disposed atthe incidence-side connecting section 8 or wavelength-multiplexed-lightconnecting section to use the incidence-side connecting section 8 as anoutgoing-side connecting section. Through this arrangement, the opticaldemultiplexer 1 can act as an optical multiplexer capable ofmultiplexing monochromatic lights of wavelengths λ1, λ2, λ3 into awavelength multiplexed light.

The optical demultiplexer according to the first embodiment can readilyseparate a light of a specific wavelength from a wavelength multiplexedlight or can readily multiplex lights different in wavelength, with asimple structure without any complicated structure and opticalcomponent. In addition, this stricture can be eliminate the need forusing an element for branching light, such as a star coupler, to preventthe degradation in light intensity.

While the optical demultiplexer according to the first embodiment hasbeen constructed to pick up all monochromatic lights included in awavelength multiplexed light, the optical demultiplexer of the presentinvention may be designed to pick up at least one monochromatic lightsincluded in a wavelength multiplexed light.

An optical filter (optical switch) according to a second embodiment ofthe present invention will be described below. FIG. 8 is a schematicperspective view showing the optical filter (optical switch) 51according to the second embodiment of the present invention. As shown inFIG. 8, the optical filter 51 comprises a light transmission section 54mounted on a base 52. The optical filter 51 further includes anincidence-side optical fiber 56 disposed on the one of the axial ends(incidence-side, hereinafter referred to as “first axial end”) of thelight transmission section 54 to enter a wavelength multiplexed light, aprism 58 disposed on the other axial end (outgoing side, hereinafterreferred to as “second axial end”) of the light transmission section 54,an outgoing-side optical fiber 60 connected to the prism 58, and atemperature controller 62 for adjusting the temperature of the lighttransmission section 54. The incidence-side optical fiber 56 isconnected to the light transmission section 54 in parallel with the axisof the light transmission section 54.

In the second embodiment, a Peltier element is used as the temperaturecontroller 62. While the Peltier element is desirable because it caneffectively perform heating and cooling, any other suitable heatingmeans, such as electric heater, or cooling means may be used.

Preferably, in order to uniformly heat or cool the light transmissionsection 54, the optical filter 51 particularly the light transmissionsection 54, is sealed with another resin, or the light transmissionsection 54 is contained in the temperature controller 62 formed in atubular shape.

The light transmission section 54 has an substantially cylindricalshape, and serves as a graded index type light transmission sectionwhose refractive index is continuously reduced in the direction from theaxis c to the periphery thereof In the second embodiment, the refractiveindex n of the light transmission section 54 has a distributionapproximated by the following formula (1):n(r)=n ₀×(1−g ² ×r ²/2)   (1),wherein n₀: refractive index on the axis c,

-   -   g: refractive index profile constant, and    -   r: radial distance from the axis c.

The light transmission section 54 is an optical component made ofinorganic glass through an ion exchange process, or made of plastic, andthe periphery of the light transmission section 54 is covered with acladding layer (not shown). The light transmission section 54 isdesigned to allow a light entered from the incidence-side optical fiber56 through the first axial end (right side in FIG. 8) to be propagatedthrough the inside thereof in a meandering manner as indicated by thearrow A in FIG. 8 and emitted from the second axial end (left side inFIG. 8).

When the entered light is a wavelength multiplexed light 64, a slightlydifferent refractive index profile will be given to each ofmonochromatic lights different in wavelength, which are included in thewavelength multiplexed light, due to the chromatic aberration and thewavelength dispersion caused by the refractive index no and therefractive index profile constant g. Thus, the monochromatic lights 66,68, 70 of the wavelengths (λ1, λ2, λ3) included in the wavelengthmultiplexed light 64 entered into the light transmission section 54 willbe propagated through the light transmission section 54, respectively,in different meandering cycles (paths). Therefore, the wavelengthmultiplexed light 64 formed by multiplexing the monochromatic lights ofthe wavelengths (λ1, λ2, λ3) and entered into the first axial end of thelight transmission section 54 are demultiplexed into the monochromaticlights 66, 68, 70 based on their wavelengths, and the demultiplexedmonochromatic lights are emitted, respectively, from the differentpositions in the second axial end of the light transmission section 54at different angle.

A refractive index is varied depending on temperature. In the lighttransmission body 54 having a cylindrical body, the temperaturedependence (variation with respect to temperature) of refractive indexvaries along the radial direction, and thereby the refractive indexprofile constant g varies depending on temperature. Thus, the outgoingpositions where the monochromatic lights are emitted from the lighttransmission section 54 can be changed by varying the temperature of thelight transmission section 54.

The variation in refractive index is apt to be dependent on the thermalexpansion coefficient of a material. A light transmission section madeof plastic can be preferably used to obtain a large variation ofrefractive index by a small amount of change in temperature. Further,the difference in thermal expansion coefficient of the material betweenthe central and peripheral regions of the light transmission section canbe preferably increased to obtain a large variation of refractive indexprofile constant by a small amount of change in temperature. Preferably,the refractive index profile of the light transmission section 54 has atemperature dependent constant of 5×10⁻⁵ or more, as described later indetail.

The optical filter 51 according to the second embodiment is designedsuch that when the light transmission section 54 is set at a firsttemperature by the temperature controller 62, the monochromatic light 66of the first wavelength (λ1) included in the wavelength multiplexedlight 64 entered into the light transmission section 54 is entered intothe outgoing-side optical fiber 60. The optical filter 51 is alsodesigned such that when the light transmission section 54 is set at asecond temperature by the temperature controller 62, the monochromaticlight 68 of the second wavelength (λ2) included in the wavelengthmultiplexed light 64 entered into the light transmission section 54 isentered into the outgoing-side optical fiber 60, as shown in FIG. 8.Further, the optical filter 51 is also designed such that when the lighttransmission section 54 is set at a third temperature by the temperaturecontroller 62, the monochromatic light 70 of the third wavelength (λ3)included in the wavelength multiplexed light 64 entered into the lighttransmission section 54 is entered into the outgoing-side optical fiber60. That is, in the optical filter 51 according to the secondembodiment, the wavelength of the light to be entered into theoutgoing-side optical fiber 60 can be selectively varied by setting thelight transmission section 54 at either one of the first, second andthird temperatures using the temperature controller 62.

A glass optical fiber made of silica glass, and a plastic optical fibermade of material such as polymethylmethacrylate, polystyrene orpolycarbonate can be used for the optical fibers 56, 60 to be connectedto the light transmission section include

The optical fibers 56, 60 are not limited to one having a specificrefractive index profile profile, but may be a conventional opticalfiber, such as step-index (SI) type of optical fibers or graded-indextype (GI) optical fibers.

The optical filter according the second embodiment can readily separatea light of a specific wavelength from a wavelength multiplexed light,and can readily vary the wavelength to be separated, with a simplestructure, without any complicated structure and optical component.

The present invention is not limited to the above embodiments, butvarious modifications and alternations can be made without departingfrom the sprit and scope of the present invention defined only by theappended claims.

With reference to FIGS. 9 to 11, an optical switch according to a thirdembodiment of the present invention. FIG. 9 is a schematic top plan viewshowing the optical filter 151 according to the third embodiment of thepresent invention. Except that no prism is provided on the outgoingside, the optical filter 151 essentially has the same structure as thatof the optical filter 51 according to the second embodiment. Thus, thecomponent corresponding to that of the optical filter 51 is defined by areference numeral derived from adding 100 to the reference numeral ofthe corresponding component of the optical filter 51.

In the optical filter 151, a silica glass single-mode optical fiberserving as an incidence-side optical fiber 156 is connected to one(hereinafter referred to as “first axial end”) of the axial ends of agraded index type plastic light transmission section 154 with 1 mmdiameter and 8.73 mm length, at the position away from the axis c of thelight transmission section 154 at a distance r of 0.45 mm and inparallel with the axis c. A Peltier element is used as a temperaturecontroller 162. A wavelength multiplexed light 164 to be entered fromthe incidence-side optical fiber 156 is formed by multiplexingmonochromatic lights having wavelengths λ1=1285.4 and λ2=1523.6. Whenthe temperature of the light transmission section 154 is 20° C., therefractive index profile constant g1 of the light transmission section154 for the wavelength λ1 is 0.543, and the refractive index profileconstant g2 of the light transmission section 154 for the wavelength λ2is 0.540. The refractive index n1 on the axis c for the wavelength λ1 is1.497, and the refractive index n2 on the axis c for the wavelength λ2is 1.493.

A light matrix in the light transmission section 154 having therefractive index profile approximated by the formula (1) is expressed bythe following formula (2): $\begin{matrix}{{\begin{pmatrix}{r2} \\{\theta 2}\end{pmatrix} = {\begin{pmatrix}{\cos({gZ})} & {{\sin({gZ})}/{ng}} \\{{- {ng}} \cdot {\sin({gZ})}} & {\cos({gZ})}\end{pmatrix}\begin{pmatrix}{r1} \\{\theta 1}\end{pmatrix}}},} & (2)\end{matrix}$,wherein n1: refractive index on the axis c,

-   -   g1: refractive index profile constant,    -   Z: length of the light transmission section 154,    -   r1: distance between the incidence position where a light P is        entered into the first axial end of the light transmission        section 154, and the axis c,    -   θ1: incidence angle (rad) at which the light P is entered into        the first axial end of the light transmission section 154,    -   r2: distance between the outgoing position where the light P is        emitted from the second axial end of the light transmission        section 154, and the axis c,    -   θ2: outgoing angle (rad) at which the light P is emitted from        the second axial end of the light transmission section 154 (see        FIG. 10).

As seen in the formula (2), the outgoing position of the light in theoutgoing end (second axial end) is changed depending on the refractiveindex profile constant g and the refractive index n. Thus, as shown inFIG. 9, when the wavelength multiplexed light 164 including theplurality of lights different in wavelength (λ1, λ2) are entered fromthe same incidence position (same radial position) in the first axialend of the light transmission section 154, the lights 166, 168 aredifferent in the outgoing position and the distance r21, r22 from theaxis c, because the refractive index profile constant g and therefractive index n are different with respect to each of thewavelengths.

In the third embodiment, under the above temperature condition, themonochromatic light of the wavelength λ1=1285.4 nm, and themonochromatic light of the wavelength λ2=1523.6 nm are emitted,respectively, from the positions r21=0.01 mm and r22=0.00 mm, and onlythe monochromatic light 168 of the wavelength λ2 is entered into anoutgoing optical fiber 160.

FIG. 11 is a graph showing the temperature dependence of the refractiveindex profile of the light transmission section 154. As seen in FIG. 11,in the light transmission section 154, the temperature dependencecoefficient of the refractive index profile constant is 6.0×10⁻⁵/° C.Thus, if the light transmission section 154 is set at a temperature of70° C., the refractive index profile constants g1 and g2 for thewavelengths λ1 and λ2 are varied, respectively, to 0.540 and 0.537, andthe distances r2 and r22 between the axis c and each of the wavelengthsλ1 and λ2 are varied, respectively, to 0.00 mm and −0.01 mm. Thus, whilethe light to be entered into the outgoing-side optical fiber 160 is themonochromatic light of the wavelengths λ2 when the temperature of thelight transmission section 154 is 20° C., it will be switched to themonochromatic light of the wavelengths λ1. In this way, the wavelengthof light to be picked up can be varied by varying the temperature of thelight transmission section 154 using the temperature controller 162.

As mentioned above, the present invention can provide an opticalmulti/demultiplexer having a reduced number of components and allowingthe number of assembling steps to be reduced while eliminating the needfor complicated adjustments.

Further, the present invention can provide an optical filter capable ofbeing readily produced and varying the wavelength of a monochromaticlight to be picked up.

1. An optical multiplexer comprising: a graded index type lighttransmission section whose refractive index is continuously reduced inthe direction from the center to the periphery thereof; awavelength-multiplexed-light connecting section disposed at one of theaxial ends of said light transmission section; and at least onemonochromatic light connecting section disposed at the other axial endof said light transmission section, wherein saidwavelength-multiplexed-light connecting section and said monochromaticlight connecting section are disposed relative to said lighttransmission section in such a positional relationship that amonochromatic light included in a wavelength multiplexed light enteredfrom said wavelength-multiplexed-light connecting section into said oneend of said light transmission section is emitted from the other end ofthe light transmission section at a position where said monochromaticlight connecting section is disposed.
 2. The optical multiplexer asdefined in claim 1, wherein said light transmission section is made ofplastic.
 3. The optical multiplexer as defined in claim 2, wherein thematerials of said light transmission section are different in Abbenumber at 20 or more.
 4. The optical multiplexer as defined in eitherone of claims 1 to 3, wherein said light transmission section has ansubstantially cylindrical body wherein said wavelength-multiplexed-lightconnecting section is disposed to the radially peripheral region of saidlight transmission section and in parallel with the axis of said lighttransmission section.
 5. (canceled)
 6. The optical multiplexer asdefined in either one of claims 1 to 3, wherein said monochromatic lightconnecting section is provided in a plural number.
 7. An opticaldemultiplexing method comprising: entering a wavelength multiplexedlight formed by multiplexing a plurality of monochromatic lights, fromone of the axial ends of a graded index type light transmission sectionwhose refractive index is continuously reduced in the direction from thecenter to the periphery thereof; demultiplexing said wavelengthmultiplexed light into said monochromatic lights in said lighttransmission section; and emitting said monochromatic lights,respectively, from different positions of the other axial end of saidlight transmission section.
 8. An optical multiplexing methodcomprising: entering a plurality of monochromatic lights, respectively,from different positions of one of the axial ends of a graded index typelight transmission section whose refractive index is continuouslyreduced in the direction from the center to the periphery thereof;multiplexing said monochromatic lights into a wavelength multiplexedlight in said light transmission section; emitting said wavelengthmultiplexed light from the other axial end of said light transmissionsection.
 9. An optical filter comprising: a graded index type lighttransmission section whose refractive index is continuously reduced inthe direction from the center to the periphery thereof; awavelength-multiplexed-light connecting section disposed at one of theaxial ends of said light transmission section; and a monochromatic lightconnecting section disposed at the other axial end of said lighttransmission section. 10-12. (canceled)
 13. The optical filter asdefined in claims 8 or 9, wherein said light transmission section has ansubstantially cylindrical body, wherein saidwavelength-multiplexed-light connecting section is connected to saidlight transmission section in parallel with the axis of said lighttransmission section.
 14. The optical filter as defined in claim 8,wherein said temperature varying means includes a Peltier element. 15.The optical multiplexer as defined in either one of claims 4, whereinsaid monochromatic light connecting section is provided in a pluralnumber